In general, packaging can significantly affect, if not dominate, the reliability of the packaged integrated circuit. In detail, an integrated device is usually fabricated on a semiconductor substrate and covered with a passivation film structure for preventing the integrated circuit from damages caused by, for example, water and impurities. A typical example of the passivation film structure is shown in FIG. 1 of the drawings. The passivation film structure comprises a phosphosilicate glass film 1 covering a silicon substrate 2 where the integrated circuit is formed, an organic insulating film 3 of polyimide and a waterproof film 4 of a silicon nitride. Though not shown in the drawings, the integrated circuit is accompanied by a plurality of bonding pads of aluminum, and a bonding wire of aurum is connected to each of the bonding pads for providing an interconnection between the integrated circuit and a leadframe. The wire bonding technique is well known as a nail head thermocompression wire bonding process in which the bonding wire is bonded to the bonding pad under thermocompression at a temperature ranging between 350 degrees and 375 degrees in centigrade. The reason why the bonding operation is carried out at the high temperature is to prevent the bonding wire from separation and, accordingly, to enhance the production yield of the semiconductor device.
However, a problem is encountered in the prior-art thermocompression process in that the waterproof film 4 tends to separate from the organic insulating film 3, which degrades the reliability of the semiconductor device. This is because of the fact that low temperature processes lower than 320 degrees in centigrade are applied to the structure in later fabrication stages but the wire bonding process is carried out at the higher temperature. This higher temperature results in that a gaseous mixture mainly composed of water is diffused from the organic insulating film 3 and, accordingly, lifts up the waterproof film 4 to produce a closed space 5 as illustrated in FIG. 1.